Preemergence efficacy studies were conducted in 1988 and 1989 to evaluate the potential of methazole (Probe^®)/norflurazon (Zorial^®) combinations in the control of DNA resistant weed species in west Texas cotton. Paired studies were conducted on annual morningglory (Ipomoea spp.), lanceleaf sage (Salvia reflexa), prairie sunflower (Helianthus petiolaris), and annual black nightshade (Solanum nigrum). Overall average of all paired weeds, locations, years, and evaluation timings showed that methazole at 0.5, 1.0, 1.5, and 2.0 lb a.i./A provided 35, 52, 61, and 70% control of the DNA resistant species tested. Methazole plus norflurazon at 0.75 plus 0.5 lb a.i. provided 61% overall weed efficacy. Methazole plus norflurazon at 1.0 plus 0.5 lb a.i. proved 74% efficacy. The 1.5 plus 0.5 lb a.i. rate of methazole plus norflurazon provided 80% overall DNA resistant weed efficacy. Norflurazon alone at 0.5 lb a.i. provided 59% efficacy. Prometryn applied at 1.2 lb a.i./A provided 70% DNA resistant weed efficacy. Diuron at 0.8 lb a.i. provided 60% DNA resistant weed efficacy. A long-term study was established to determine the effects of rate, application method, and repeated applications of norflurazon on soil persistence and plant injury for sorghum and wheat. Preplant incorporated (PPI) and preemergence (PRE) applications of norflurazon at 1.0 (lX) and 2.0 (2X) lb a.i./A. were applied. Subplots were retreated for two and three years. Evaluations taken 14 months after initial treatment showed slight injury to wheat and sorghum from 1X applications and significant injury from 2X applications. Evaluations 1 to 3 years after final application indicated that repeated applications, even at the IX rate, increased norflurazon residues and rotational crop injury. At both 1X and 2X rates, PPI treatments increased norflurazon persistence compared to PRE applications. An additional long-term norflurazon study was established to evaluate the effects of 0.5 lb a.i./A (0.5X) applied both PPI and PRE. This study is being conducted in the same method as the previous study. Early wheat evaluations (5 months after application) indicate again that the PPI application is more injurious than the PRE application. The 0.5 lb a.i./A rate (0.5X) of norflurazon (spring 1989 applied) is providing only minor wheat damage. Sorghum and corn will be planted in the spring of 1990, 1991, 1992.